Vertical steel column box-sections supporting a staircase on a UK North Sea platform had been almost completely destroyed by corrosion.
The load capacity of the box-sections had been severely reduced, and there was little of the box-section profile remaining. In total, over 20 linear meters of steel columns had to be repaired.
The CompoSol® Structure repair was required to completely restore the loading capacity of the staircase, assuming no contribution from any remaining steel. The connections between the horizontal beams and vertical columns were also damaged, thus the solution would have to transfer the vertical reactions from the beams to the columns.
The CompoSol® Structure repair consisted of pre-fabricated GFRP sections bonded to the remaining steel substrate, which gave a smooth surface onto which CFRP was applied, in order to provide the requisite load resistance. The connection points were repaired with CFRP plates and angles bonded with a high-strength CompoSol® adhesive. The entire repair was then wrapped with layers of GFRP in order to consolidate the repair and increase the impact resistance.
The requirement of the CompoSol® Structure repair to carry all the vertical loads induced by the staircase meant that the design stage was vital. The key aspect was ensuring that the CompoSol® Structure repair adequately carried the axial compressive loads and bending moments.
The columns were slender sections, thus buckling due to compressive loads was the main concern. First principles were used to calculate the required cross-sectional area of the repair when subjected to buckling and bending loads. Continuous layers of uni-directional carbon bonded along the axis of the columns provided the bending and buckling resistance; meanwhile shear stresses were carried by layers of CFRP laid at +/- 45° to the axis of the column. Hand calculations were checked using Finite Element Analysis (FEA) software.
The vertical reactions from the horizontal beams were resisted by CFRP plates designed to transfer the shear stresses induced by the vertical reactions. A build to order high-strength adhesive (CompoSol® CRC) was required to ensure that the plates did not undergo lap-shear failure.
The main advantages of the CompoSol® Structure solution were:
- The absence of hot work, heavy materials, or machinery meant that the platform could continue day-to-day operation with very little disruption;
- The amount of material required compared to a complete steel replacement meant that the cost of the composite repair was a lot lower;
- Three men took 21 days to complete 20 linear meters of repair.